The present invention relates to the field of hydrocarbon processing.
Many refineries recover valuable products from the heavy residual oil that remains after refining operations are completed. This recovery process, known as delayed coking, produces valuable distillates and coke in one or more large vessels known as coke drums or coking vessels. As used herein, coking drums and vessels are used interchangeably.
Coke drums are typically large, cylindrical vessels having a top head and a frusto-conical bottom portion fitted with a bottom head. Coke drums are usually present in pairs so that they can be operated alternately. Thus, while one coke drum is being filled with residual oil and heated, the other drum is being cooled and purged of up to several hundred tons of coke formed during the previous recovery cycle. The operating conditions of delayed coking can be quite severe. Normal operating pressure typically range from 40 to about 60 pounds per square inch, and the feed input temperature may be over 900.degree. F.
Coke recovery begins with a water quench step in which steam and water are introduced into the coke filled vessel to complete the recovery of volatiles and to cool the mass of coke. The vessel is then vented to atmospheric pressure and the top head (typically a 4-foot diameter flange) is unbolted and removed. A hydraulic coke cutting apparatus is inserted into the vessel to cut the coke, and finally, the bottom head (typically a 7-foot diameter flange) is unbolted and removed to allow the hydraulically cut coke to fall out of the vessel and into a recovery chute. While ideally all of the coke is captured by the chute and falls through to a coke pit below, in practice there is spillage, and essentially all of the coke is taken to be channeled through to the coke pit as long as no more than a few percent of coke spills onto the floor.
Due to the size of the drum bottom outlet, the large mass of coke and water which falls through the outlet, and the need to position the drum bottom outlet away from the floor so that the drum bottom head can be removed, positioning the recovery chute with respect to the coke drum bottom outlet can be difficult. In a typical installation, the coke chute is located in a channel between the switch-deck floor and a coke pit below. Once the coke drum head is removed, or at least pivoted or otherwise moved out of the way of the drum outlet, chain hoists are attached to the circumference of the chute, and the chute is manually raised to mate with the coke drum bottom flange. This process may be dangerous in that operators involved with raising of the chute may be injured during a coke cave-in within the drum.
One possible solution, described in U.S. Pat. No. 4,960,358 to DiGiacomo, issued Oct. 2, 1990, is to suspend the chute from the drum using hydraulic cylinders, and then to use the hydraulic cylinders to raise and lower the chute. While this serves to automate movement of the chute, it is still unsatisfactory in that the cylinders are positioned above the floor where they can interfere with the deheading process by impeding access to the coke drum flange. Other difficulties may also exist, in that the DiGiacomo device still requires insertion of a gasket between the chute to the bottom flange, and the device forms part of an integrated deheading frame device, which may render it inapplicable to installations using alternative deheading systems.
Thus, there is a further need for an improved method and system of positioning a coke chute with respect to the bottom opening of a coke drum. Other and further objects and advantages will appear hereinafter.